1. Field of The Invention
The invention relates to a method for the manufacture of multilayered contacts for medium-voltage vacuum power circuit breakers.
2. Description Of The Prior Art
Formerly, contact materials for medium-voltage vacuum power circuit breakers comprised alloys with a copper base. It has been found, however, that it is advantageous to use composite materials with burn-off-reducing and getter components to lengthen contact life and to promote residual gas adsorption. The contact material should have a smoothly-fused surface after a switching stress and must not have a tendency to form points, droplets, or loose fragments in order to ensure maximum dielectric strength after power interruptions. It has been found that these requirements can be met simultaneously by a material which consists of copper contained in a composite material consisting of at least 35% by volume of chromium or vanadium and the remainder consisting of chromium, vanadium, cobalt, nickel or iron.
The addition of metals to the composite material with melting points below 1400.degree. C is not desirable because the resulting burn-off resistance is too low. Metals with a melting point of over 2000.degree. C, on the other hand, no longer lead to smoothly-fused surfaces in every case after a switching stress thereby adversely affecting the dielectric strength. Other metals having melting points between 1400.degree. C and 2000.degree. C, such as zirconium and titanium, cannot be used in the contact layer because they form low-melting inter-metallic compounds of low burn-off resistance with the copper phase.
The contact materials mentioned above are produced by the well-known vacuum-sintering impregnating method, wherein a composite body of metal powders, usually a pressed powder blank or a sintered body, of the metals is impregnated with liquid copper at a temperature above the melting point of copper in a crucible under vacuum. Considerable difficulties arise if the impregnating crucible containing the composite materials and the copper, or the impregnating supports on which composite materials are placed, are of large area because the reactive metals contained in the composite materials, particularly chromium and vanadium, react with all known materials used in such crucibles or supports. Due to this reaction (called set-in of alloying), undesired components of the crucible or support material are introduced into the contact layer material by diffusion.
To avoid the above difficulties, the impregnating can also be done without a crucible by using supports with a small area rather supports with a large area. With this method, however, a change occurs in the boundary surface tension of the impregnating copper at the support points of the contact blank, unavoidably resulting in segregation of the copper and underimpregnation of the composite material.
A further difficulty arises when joining the contacts to the support studs because the reactive metals contained in the contact, such as chromium and vanadium, lead to oxidation of the contact surface because of their high oxygen affinity, even when soldering takes place in a vacuum, and thereby prevent wetting by the commonly-used solder metals. To avoid these difficulties, the contact is provided with an electrodeposited metal layer of, for example, nickel or copper, prior to being soldered. With this method however the danger exists that the contact may become contaminated with electrolyte residue. There are several known special solders, which contain phosphorus or maganese as deoxidizing components, which permit soldering without an electrodeposited layer, but these solders are not usable for vacuum power circuit breakers because of the low boiling point of the deoxidizing additives.